A GaN based compound semiconductor crystal (InxGayAl1-x-yN, where 0≦x, y; x+y≦1), such as GaN, InGaN, AlGaN, InGaAlN, is expected as a material of a semiconductor device, such as a light emitting device, a power device, and is noticed as a material which can be applied in other various fields.
In an earlier development, because it was difficult to grow a bulk crystal of a GaN based compound semiconductor, a substrate made by heteroepitaxially forming a thin film single crystal, such as GaN, on a different single crystal, such as sapphire, was used for the above electronic device.
However, because a lattice mismatching between a sapphire crystal and a GaN based compound semiconductor crystal is large, there was a problem that the dislocation density of the GaN based compound semiconductor crystal grown on the sapphire crystal is large and a crystal defect is caused. Further, because the sapphire has a low heat conductivity and it is difficult to release heat, there was a problem that when a substrate made by growing a GaN based compound semiconductor crystal on the sapphire crystal was used for an electronic device or the like, in which consumption power was large, a temperature thereof easily became high.
A substrate having a large heat conductivity and having the lattice matching with the GaN based compound semiconductor crystal, is strongly required. The ELO (Epitaxial lateral overgrowth) method using the hydride vapor phase epitaxy (hereinafter, abbreviated to HVPE), or the like has been rapidly researched. In this case, the ELO method is a method in which an insulting film is formed as a mask, for example, on a sapphire substrate and the insulting film is used as a mask by providing an opening portion on a part of the insulting film. Then, a high crystalline GaN based compound semiconductor crystal is grown by using the exposed sapphire substrate surface as a seed for the epitaxial growth. According to the method, because the growth of the GaN based compound semiconductor crystal is started from the sapphire substrate surface in the opening portion provided on the mask and the growth layer is expanded on the mask, the dislocation density in the crystal can be suppressed. It is possible to obtain the GaN based compound semiconductor crystal having little crystal defect.
However, because the GaN based compound semiconductor crystal obtained by the ELO method has a large heat strain, there was a problem that the GaN based compound semiconductor crystal wafer is strained when the sapphire substrate is removed by the polishing in a wafer manufacturing process to obtain the GaN based compound semiconductor crystal wafer.
The inventors proposed a method for heteroepitaxially growing the GaN based compound semiconductor crystal by using a rear earth 13 (3B) group of perovskite crystal as a material of a different single crystal substrate and by using {011} face or {101} face thereof as a growth surface (WO95/27815). In this case, {011} face or {101} face is equivalent to (011) face or (101) face, respectively.
According to the growing technology in the prior application, when GaN is grown, for example, on the {011} face or the {101} face of NdGaO3 which is one of the rear earth 13 (3B) group of perovskites and is used as a substrate, the lattice mismatching is decreased to about 1.2%. The value of the lattice mismatching is relatively low as compared with the value of the lattice mismatching in case that a sapphire or SiC used as a substitution thereof is used as a substrate. Because the dislocation density in the crystal is low, it is possible to grow the GaN based compound semiconductor crystal having little crystal defect.
However, when a GaN based compound semiconductor crystal was grown on the NdGaO3 substrate by using the growth method in the prior application, it was found that cracks, fracture, warp or the like are caused in both the NdGaO3 substrate and the grown GaN crystal. In particular, when the cracks were caused in the GaN crystal, it was not possible to obtain a large and excellent GaN thick substrate. Therefore, the most serious problem was caused.
An object of the present invention is to provide a method for preparing GaN based compound semiconductor crystal, in which it is prevented by carrying out a reducing and decomposing process for a substrate before a cooling step to remove the substrate that cracks are caused in the GaN based compound semiconductor crystal.